Definition
An isobar is an imaginary line on a map that connects points with the same air pressure. On a weather forecast, it's the most basic and important tool for spotting high- and low-pressure areas and predicting where strong winds will blow.
Isobars draw the 'height' of the air, like contour lines
Open a hiking map and you'll see contour lines, loops that connect points of equal ground height. The squiggly lines on a weather map work on exactly the same idea. The only difference is that instead of the height of a hill, they use air pressure, the weight of the air pressing on the ground.
Air is completely invisible, but it has real weight, and it's constantly pressing down on your head. We call that push air pressure. In some places cold air packs together and gets heavy. In others, warm air rises and gets light.
Meteorologists measure the pressure at weather stations across the country, then connect the places with the same reading. Once the lines are drawn, the invisible ups and downs of the air appear like magic. Heavy, piled-up air (high pressure) becomes a bulging hill, and thin air (low pressure) becomes a sunken valley of air.
Look at those air hills and valleys and you can easily guess which way the air will move next. An isobar map lays out the terrain of a huge, invisible ocean of air on paper so you can see it at a glance.
The closer the lines, the stronger the wind
Put a ball at the top of a steep slide and it rolls down fast. Put it on a gentle hill and it rolls along lazily. Air behaves the same way. It always slides from a heavy, packed high-pressure hill toward a light, loose low-pressure valley. That's the wind you feel on your skin.
When isobars are packed close together, it means the pressure changes a lot over a short distance. In other words, the slide is very steep. A steep slide sends air rushing down, so you get strong, dangerous winds like those in a hurricane.
If the isobars are spread far apart, the air hill is very gentle. The pressure difference is small, so the air drifts slowly, and you get a light breeze or calm, quiet weather.
That's why, when a hurricane approaches, the isobars wrap tightly around its center in ring after ring, like tree rings. If you spot an area on a weather map where the lines are crowded together, people there need to get ready for strong winds.
To be more precise, the values are adjusted to sea level
When you climb a tall mountain, the column of air above you gets shorter, so the air pressure naturally drops. If we plotted the pressure measured at the top of Mount Rainier straight onto a map, every mountaintop would look like a powerful low-pressure storm zone, no matter what the weather was really doing.
To remove that terrain distortion, weather services recalculate every station's reading to sea level, the height of the ocean's surface. Even for a number measured on a peak, they work out how heavy the air would be if that spot were a flat beach. This correction is called a sea-level pressure reduction.
Using pressures adjusted to the same level, meteorologists around the world usually draw a line at every 4 hectopascals (hPa). Lines at multiples of 20, such as 1000 hPa or 1020 hPa, are sometimes drawn a bit thicker so they're easy to spot.
Thanks to this, we aren't fooled by rugged terrain, and we can clearly see the flow of the real weather systems that actually stir the Earth's atmosphere.
Common misconceptions
Wind blows straight from high pressure to low pressure, crossing the isobars at a right angle.
Because of the Coriolis effect, caused by the Earth's rotation, wind can't cross isobars in a straight line. High up, where there's almost no friction, wind blows nearly parallel to the isobars. Near the ground, friction slows the wind, so it crosses the isobars at an angle toward low pressure.
Where you meet it
On a hurricane forecast map, you see isobars packed tightly around the storm and prepare for wind damage.
When you're near the center of a high-pressure area with widely spaced, rounded isobars, you can expect clear skies and light wind.
In one sentence
An isobar is a line connecting places with equal air pressure, and the spacing between the lines tells you how strong the wind is and how the weather will change.